1. Field of the Invention
This invention relates to a multilayer magnetic recording medium and, in particular, to a dual layer magnetic recording tape having improved magnetic recording properties.
2. Description of the Prior Art
Recently, with the requirements of quality improvement and high density of magnetic recording tapes in both open reel tapes and cassette tapes, multilayer type magnetic recording tapes, that is, magnetic recording tapes having two or more magnetic recording layers, have been developed.
Multilayer magnetic recording tapes are described in, for example, Japanese Patent Publication Nos. 2218/'62 and 23,678/'64; Japanese Patent Application (OPI) Nos. 31,602/'72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") (or U.S. Pat. No. 3,761,311); 31,804/'72; 31,907/'73 (or U.S. Pat. No. 3,775,170); and 31,804/'75; U.S. Pat. Nos. 2,643,130; 2,647,954; 2,041,901 and 3,676,217; and West German Patents (DT-AS) Nos. 1,190,905 and 1,238,072.
In this case, an important problem is the setting of the operating bias and equalization and the setting value is usually very near a generally designated standard bias. Therefore, magnetic recording tapes operable at this value are readily and most generally used and thus are very advantageous since such magnetic recording tapes do not require any specific bias setting and equalization.
As the easiest means for increasing the magnetic recording density, increasing the coercive force of the ferromagnetic iron oxide used for the magnetic recording tapes has been proposed. However, such magnetic recording tapes are not generally compatible with the above-described standard bias in using these tapes and in order to obtain optimum characteristics for magnetic recording tapes, a different operating bias and equalization setting for the magnetic recording tapes must be used. That is, a standard bias, a chromium dioxide bias, a multilayer magnetic tape bias, etc. are used.
Conventional dual layer magnetic recording tapes may be superior in the output in the low frequency region but they require a specific position in operating bias and equalization.
This is, for example, as shown in Table 1 below.
TABLE 1 ______________________________________ Type of Magnetic Bias.sup.(1) Equalizer.sup.(2) Recording Tape (%) (.mu.sec.) ______________________________________ Low-Noise Type 100 120 Fe-Cr Type.sup.(3) 130 35-50 CrO.sub.2 Type 160 70 ______________________________________ .sup.(1) Low noise type magnetic recording tape is shown as standard (100%). .sup.(2) The time constant (.mu.sec.) of low noise type equalization is shown as a standard (120 .mu.sec.). The time constant of a FeCr type magnetic recording tape is usually 40-60% of the standard value and that of a CrO.sub.2 type magnetic recording tape is usually 50-70% of the standard value. .sup.(3) DUAD Ferri Chrome tape (registered trade name, made by SONY Corp.), Scotch CLASSIC Cassette tape (registered trade mark, made by 3M Co.), Ferrochrome tape (registered trade name, made by BASF A.G.), and CARAT tape (registered trade mark, made by AGFAGEVAERT N.V.) are used as examples.
Accordingly, in order to use tape recorders, tape decks, etc., under the best conditions for a magnetic recording tape, the devices must be equipped with the bias and equalization positions as shown in Table 1 above.
Conventional low-noise type and CrO.sub.2 type magnetic recording tapes which are known to have the best characteristics have, for example, the fundamental properties of Sample No. 1 and Sample No. 2 shown in Table 4 below. Furthermore, multilayer magnetic recording tapes developed for the purpose of further improving the characteristics of magnetic recording tapes, Sample No. 1 and Sample No. 2 have the fundamental properties of, for example, Sample No. 3 and Sample No. 4 as illustrated in Table 4 shown hereinafter.
In addition, the layer structure of a dual layer type magnetic recording tape is illustrated in FIG. 1 of the accompanying drawings as a schematic enlarged sectional view thereof, wherein an inner magnetic recording layer 2 and an outer magnetic recording layer 1 are formed on a non-magnetic support 3.
In Table 4, Sample No. 3 is the dual layer magnetic recording tape prepared according to the description of Japanese Patent Application (OPI) No. 51,908/'77 (corresponding to U.S. Pat. No. 4,075,384) and Sample No. 4 is also a dual layer magnetic recording tape prepared in a similar manner except that the coercive force of each magnetic recording layer thereof was 1.6 times higher than that of each corresponding magnetic recording layer of Sample No. 3. As will be understood from Table 4, a dual layer magnetic recording tape can be provided with excellent characteristics as compared with single layer type magnetic recording tapes.
Also, multilayer (dual layer) magnetic recording tapes prepared for the purpose of further improving the sensitivity in the high frequency range for the frequency characteristics (FIG. 2 and FIG. 3) of Sample No. 3 and Sample No. 4 have the fundamental characteristics of, for example, Sample No. 5 and Sample No. 6 shown in Table 4. As is shown in FIG. 2 and FIG. 3 of the accompanying drawings, both Sample No. 5 and Sample No. 6 have higher sensitivity in the high frequency range than those of Sample No. 3 and Sample No. 4 but the sensitivity in the range of 2 KHz-6 KHz is low, which results in unbalanced sound. Furthermore, on comparing Sample No. 3 with Sample No. 5 and Sample No. 4 with Sample No. 6, it will be understood that Sample No. 5 and Sample No. 6 have a higher harmonic distortion factor, a lower maximum output level (MOL), a lower S/N ratio (signal to noise ratio), and less dynamic range. Thus, when the difference in coercive force between the inner magnetic recording layer and the outer magnetic recording layer of a dual layer magnetic recording tape is increased, the frequency characteristics may be increased but the sound balance frequently becomes poor.
Moreover, in order to record and reproduce using a magnetic recording head without reducing the magnetic characteristics of the inner magnetic recording layer of a dual layer magnetic recording tape, the outer magnetic recording layer thereof preferably is thin and in order to obtain sufficient effects such as an improvement in sensitivity in the high freqeuncy range by a thin outer magnetic recording layer, it is desirable to increase the maximum residual magnetic flux density of the magnetic recording layer.
Therefore, in using fine particles of chromium dioxide having a comparatively high coercive force for the outer magnetic recording layer of a dual layer magnetic recording tape, there are the disadvantages that it is difficult to industrially produce chromium dioxide particles having a coercive force of higher than about 700 oe and when the coercive force thereof is increased, the maximum residual magnetic flux density is reduced. Also, the use of chromium dioxide particles for the outer magnetic recording layer is undesirable since the abrasion of the magnetic recording head is severe and the life of the magnetic recording head is reduced. Still further, since thermal demagnetization of a magnetic recording layer composed of chromium dioxide particles occurs to a great extent and the balance between the magnetic characteristics of the outer magnetic recording layer and the magnetic characteristics of the inner magnetic recording layer due to changes in temperature is lost, the use of chromium dioxide particles as a material for one magnetic recording layer of a multilayer magnetic recording material is not very desirable.